The present invention relates to an improvement in a heat sensitive paper for thermal printing devices and a heat sensitive coating composition for such papers comprising a chromogenous basic triphenylmethane derivative, in particular a basic diarylphthalide derivative, and an acidic phenolic color-developer in a carrier composition as described in U.S. Pat. No. 3,539,375 to H. H. Baum.
As described in the aforementioned U.S. patent, the entire disclosure of which is herein incorporated by reference, the chromogenic compound and the phenolic colordeveloper both are distributed in finely divided solid form in a carrier or binder, preferably a polyvinylalcohol. Upon local application of heat, one of the color-forming reactants usually the phenolic compound, is fluidized thus leading to an intimate contact between the two color-forming reactants and the taking place of the color-forming reaction localized at the site of heating at thermal printing temperatures. Heat-sensitive record materials containing a chromogeneous and a color developing compound distributed in a polyvinyl alcohol coating composition are also disclosed in U.S. Pat. Nos. 3,920,510, and 3,674,535.
The use of a polyvinyl alcohol film as a matrix for heat sensitive compositions is said to provide unexpected properties which make it uniquely attractive as a temperature responsive record material. It is widely used in conventional heat sensitive imaging systems and compositions. However, such conventional heat-sensitive coating compositions and papers for thermal printing coated therewith still suffer from various disadvantages, such as a tendency for premature self-color development, lack of environmental stability, i.e. to heat, moisture and light, as well as an undesirable degree of pressure sensitivity of the coated paper. Moreover, such compositions, as those described in U.S. Pat. No. 3,539,375, suffer the disadvantage of sticking as the paper moves past the printing head. This necessitates the addition of talc to alleviate this problem, see column 7, lines 64 and 65 of this patent.